Extreme Heat & Fertility in Nigeria

Motivation

Climate change is increasing the intensity, frequency, and severity of extreme heat events (IPCC 2023).
Impacts on fertility are understudied relative to other demographic processes but link directly to climate projections (Grace 2017; Kuehn & McCormick 2017; Sellers & Gray 2019).
Low-income, tropical countries face a triple burden (Grace 2017; Potts & Henderson 2012):
- Climate vulnerability
- High population growth
- Poor with weak health systems

Evidence on heat & fertility relationship

Extreme heat exposure adversely impacts pregnancy outcomes, such as preterm birth, low birthweight, and miscarriages (Conte Keivabu et al. 2022; Davenport et al. 2020; Kuehn & McCormick 2017; Beltran et al. 2013)
Extreme heat reduces (Sellers & Gray 2019) or delays (Barreca et al. 2018) births in some contexts
Fertility preferences also impacted by heat exposure (Sellers & Gray 2019; Eissler et al. 2019)

Gaps and methodological issues

Many studies identify potential biological and behavioral pathways:
- Biological: disease incidence, nutrition, infecundity, thermal stress
- Behavioral: replacement, coital frequency, labor migration, income shocks
Critical exposure windows
Acute vs. chronic exposure and acclimatization
Differential impact of humid heat
Definitions of extreme heat exposure

In this paper…

We focus on Nigeria where hot-humid heat exposure has rapidly increased and fertility is high (avg parity of 4.4)
Examine effects of different definitions of extreme heat exposure on two fertility outcomes
Explore potential mechanisms

In this paper we focus on Nigeria, which is one of the most climate vulnerable countries in the world. Hot-humid heat exposure has been increasingly rapidly but there are striking geographic differences and a strong north-south gradient in heat, precipitation, and ecological zone. Nigeria also has a large and growing population – in our sample average parity is 4.4 children per woman – making it a good setting in which to study the effects of extreme heat on fertility.

We construct multiple measures of extreme heat exposure and examine 2 outcomes related to fertility: conceptions and pregnancy loss and explore several of the hypothesized mechanisms in the literature.

Specifically, by examining both conceptions and pregnancy loss, we try to separate the potential impacts on the ability to conceive from the ability to carry to term.

Data

Fertility

Two waves of Nigeria DHS with fertility calendar module (2008 & 2018)
Outcomes: conception, pregnancy loss
Restrict sample based on length of residence
Organize in “person-period” format for survival analysis

Heat

Combine CHIRTS daily Tmax record from 1986-2016 with relative humidity to approximate wet-bulb globe temperature (WBGT)
Spatially merge with DHS clusters
Construct several measures of heat exposure during each month (or trimester) of observation

We use 2 waves of the Nigerian Demographic and Health Data that contain both the fertility calendar module and collected geographic data. The fertility calendar data allows us to identify the conceptions, pregnancy losses, and live births for women in the approximately 60 months preceding the DHS interview.

We restrict the sample according to the length of time women have lived in their current residences so that we can ensure they were exposed to the heat data we are merging in.

As I’ll explain further in a moment, we will construct the data in person-period format to analyze conceptions.

To identify extreme heat, we combine the CHIRTS daily temperature record, which is the most accurate daily maximum temperature record for data-sparse regions like sub-Saharan Africa, with relative humidity to approximate wet-bulb globe temperature, which captures the effect of humidity and temperature.

We produce the heat record at a 5-degree spatial resolution and spatially merge it with the DHS data at the cluster level, which corresponds to approximately a village in rural areas or a city block in urban areas, which allows us to identify the heat that women have been exposed to over the entire duration they’ve lived in their current residence. We take a 10km buffer around each cluster centroid and area-weight the heat data.

We then construct several measures of heat exposure at the monthly level for each month of observation for the calendar and also according to trimesters for when we look at pregnancy losses.

Humid Heat Has Increased Along Geographic Gradient

Heat is correlated with conception probability

Women experience conceptions at different times and not all women will experience a conception during their recorded fertility calendar. For women who do not experience a conception during the fertility calendar window, their records are therefore right-censored by the date of the interview (the most recent month of record for the fertility calendar). This type of data is well-suited for survival or time-to-event analysis, which accounts for the censoring.

This is a kaplan-meier survival curve of the conception probability in each period, split out by heat exposure in that month. This figure shows relatively proportional probabilities of conception for women experiencing low (19 - 25.4C) and moderate (25.4 - 27.2C) heat, but for women who experience high heat (27.2 - 31.4C) the probability of conception is higher in every month of the fertility calendar after month 5.

For the analysis, we follow a discrete-time approach to model the conditional probability of conception in each month and use a logistic regression function, so we can control for individual and household level characteristics, such as age, marital status, wealth, religion, and parity, and include cluster fixed effects, month fixed effects, and year fixed effects.

Humid-heat Increases Probability of Conception

Humid-heat Increases Probability of Pregnancy Loss

Heterogeneous Patterns For Conception

Conclusion & Next Steps

Preliminary results suggest humid-heat is associated with an increase in conception and pregnancy loss (in the 1st and 2nd trimesters)
Effect is concentrated among younger, less educated, and rural women
Continue exploring heterogeneous effects for conception and pregnancy loss to better understand potential mechanisms
Explore alternative definitions of heat exposure